Needs a comprehensive answer. There may be one somewhere else already. Z.
This is from the decoder manual and does not answer the question:
A 12 or 14 Volt lamp run directly without resistors will be less sensitive to voltage fluctuations. For regular 12 to 16 Volt lamps that draw more than 50 mA when lit, we recommend that you put a 22 to 33 ohm 1/4 watt resistor in series with the lamp leads. This will ensure that the lamp "start-up currents" (up to 10 times normal current draw) do not overload the outputs.
If you want, you can estimate things by using Ohm's Law:
Amperage(I) equals volts(V) divided by resistance (R)
You would measure the voltage output of the decoder and then measure the resistance of the bulb with the Ohms segment of your multimeter. You could then determine if this bulb is correct for this installation.
Many older locomotives use bulbs that draw a lot of current and are really large.
Using an LED in place of a blub works well in most cases.
In some cases, using an incandescent bulb may produce enough heat to distort the shell.
That is the case with car lighting such as with the Kato passenger cars; they make a special LED light set for use with DCC because incandescent car light sets operate at full voltage all the time, causing the shells to warp.